1. Technical Field
The invention concerns an electronic circuit having a sigma-delta modulator for a power amplifier.
2. Discussion of Related Art
Sigma-delta modulators (also referred to as delta-sigma modulators) with downstream-connected power amplifiers are used as highly efficient amplifier systems. This involves, for example, audio amplifier systems or transmitting stations for telecommunications. Frequently the power amplifiers are class S amplifiers. With that type of amplifier the input signal is not linearly amplified. Instead power transistors are switched on and off by means of pulsed signals at a high frequency. The result at the output of the power amplifiers is thus a pulse series which contains the analog signal for the load. The terminology is not uniform on a worldwide basis: depending on the respective author involved, such amplifier types are referred to as class S or class D (see, for example, Jack R Smith: ‘Modern Communication Circuits’, 2nd edition, McGraw-Hill 1998, Chapter 11.6, page 481). Hereinafter in the description of this invention, the term ‘class S amplifier’ is used for such switched amplifiers, the pulse series of which contains information in respect of the analog signal. Class S power amplifiers make it possible to provide output signals of high power with comparatively low thermal power losses. A suitable signal for the actuation of loads (such as loudspeakers or the like) is obtained from the pulsating high-frequency signal in reconstruction filters. In the simplest case the frequency response characteristic of the load can itself form the reconstruction filter if for example the loudspeaker cannot follow the high-frequency pulses but integrates them to afford a low-frequency analog signal. In a typical case, in a sigma-delta modulator the digital signal of an A/D converter (analog-digital converter), which can be, for example, a 1-bit signal from a comparator, is converted into an analog value in a D/A converter and fed as a correction value into the regulating loop of the sigma-delta modulator. Usually in that way, for example, the contribution due to the quantisation error of the A/D converter, that is to say the quantisation noise, is reduced so that it is possible to use low-resolution A/D converters. The 1-bit output signal of sigma-delta modulators has a sampling rate which is far above the required Nyquist frequency of the input signal. Accordingly the output signal of the sigma-delta modulator is like a signal as is required for operation of the power amplifiers. Both components are therefore advantageously used together. The feedback to the input of the sigma-delta modulator can not only reduce the quantisation error or the quantisation noise of the 1-bit A/D converters in the modulators. Other transmission errors in the forward path can also be compensated by further feedback. More extensive compensation is achieved, for example, by the feedback from the analog signal being effected after the power amplifier or a subsequent reconstruction filter. Transmission errors can be compensated in a larger part of the signal path in that way. Such feedback configurations can provide very high linearity in the entire system. The specified principle is therefore applied, for example, in the area of audio signal processing in which a high degree of linearity is desired.
Often however it is difficult to obtain a signal which is exactly proportional to the output signal, particularly in the case of higher-power amplifiers in which then a voltage divider or a current sensor must be incorporated into the output path. A further difficulty is encountered if the signal frequency is very high and the power amplifier is not on a common substrate (that is to say on the same chip) with the modulator. In such a situation stability problems and errors can occur due to an unwanted delay and changes occurring elsewhere in the fed-back signal. On the other hand integration of electronic components on a single substrate is not always possible, either because they require too much area, a suitable technology does not exist or an excessive power consumption or interference would be caused, which would entail excessive disadvantages for other components on the same substrate.